Coated casting nozzles

ABSTRACT

A VITREOUS MOLTEN METAL SUPPLY DEVICE FORMED OF A REFRACTORY OXIDE MATERIAL, SUCH AS A FUSED SILICA CONTINUOUS CASTING NOZZLE, HAVING ON THE SURFACE THEREOF A THIN METAL COATING OF A THICKNESS RANGING BETWEEN 0.002 AND 0.02 INCH SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, MOLYBDNUM, NIOBIUM AND TANTALUM WHICH PROTECTS THE VITREOUS OXIDE MATERIAL AGAINST ATTACK BY A MOLTEN CASTING SLAG HAVIS PREFERABLY APPLIED BY A PLASMA SPRAY PROCESS IN ORDER IS PREFERABLY APPLIED BY A PLASMA SPRAY PROCESS IN ORDER TO AVOID DEVITRIFYING THE FUSED SILICA NOZZLE.

United States Patent Office 3,806,357 Patented Apr. 23, 1974 3,806,357COATED CASTING NOZZLES James W. Halley, Chesterton, and Norman T. Mills,Highland, Ind., assignors to Inland Steel Company, Chicago, Ill. NoDrawing. Filed Dec. 7, 1971, Ser. No. 205,699 Int. Cl. C03c 17/06; F27b3/18 US. Cl. 117-94 14 Claims ABSTRACT OF THE DISCLOSURE A vitreousmolten metal supply device formed of a refractory oxide material, suchas a fused silica continuous casting nozzle, having on the surfacethereof a thin metal coating of a thickness ranging between 0.002 and0.02 inch selected from the group consisting of tungsten, molybdenum,niobium and tantalum which protects the vitreous oxide material againstattack by a molten casting slag having a high solubility for oxides. Thethin coating of metal is preferably applied by a plasma spray process inorder to avoid devitrifying the fused silica nozzle.

The present invention relates generally to an apparatus for thecontinuous casting of molten metal and more particularly to an improvedtubular supply device for introducing molten steel into a continuouscasting mold.

In the continuous casting of a molten metal, such as steel, it has beenfound that improved casting results can be achieved by introducing themolten metal below the surface of the pool of metal maintained in theupper end of the mold. Various tubular devices have been designed forconveying and discharging molten metal into a continuous casting mold,such as the improved casting nozzle structures of the Mills et al. US.Pat. No. 3,517,726. Most of the supply nozzles used for conductingmolten steel into a continuous casting mold below the surface of thepool of metal therein are made of fused silica.

It has also been found that further improvements in casting results areobtained by providing on the surface of the molten metal pool maintainedin the upper end of the continuous casting mold a protective layer ofslag. It is important that the slag layer, in addition to reducing heatlosses and oxidation at the surface of the molten metal, also readilydissolve oxides, such as alumina, which are rejected by the molten metaland form a scum on the surface of the pool of molten metal in the mold.Many different compositions have been used to form the slag layer,including borax, sodium silicate, blast furnace slag, window glass,bottle glass and other improved synthetic slags. An example of animproved continuous casting synthetic slag composition which readilydissolves alumina is found in the Halley et al. US. application Ser. No.52,- 750, filed July 6, 1970, now Pat. No. 3,647,249.

Certain of the continuous casting slag compositions,

particularly those which have a high solubility for alumina, alsorapidly attack refractory oxide nozzles, such as the fused silicanozzles. In many cases the wall of the nozzle is eaten away orcompletely dissolved at the Slag line within the mold during the castingoperation with the result that the quality of the remainder of thecasting is poor.

It is, therefore, an object of the present invention to provide a moltenmetal supply device for the continuous casting of metal which isresistant to attack by slags having a high solubility for oxides. 1

It is a further object of the present invention to provide a metalcoated fused silica supply device for conveying molten metal into acontinuous casting mold which is highly resistant to attack by acontinuous casting slag having a high solubility for silica.

It is still another object of the present invention to provide animproved method of increasing the resistance of a fused silica moltenmetal supply device to attack by a continuous casting slag having a highsolubility for silica.

Other objects of the present invention'will be apparent from thedetailed description and claims to follow.

It has been found that a molten metal supply device made of a refractoryoxide, such as a fused silica nozzle, can be provided with increasedresistance to attack from a continuous casting molten slag compositionwhich has a high solubility for alumina (i.e., at least 20% by wt.),silica, and the like refractory oxides and which normally rapidlyattacks a fused silica nozzle by applying to the nozzle on at least thesurfaces thereof which come in contact with the molten slag layer in themold an adherent surface coating or film of a metal having a low vaporpressure at steel casting temperatures (i.e., up to 3100 F.), a meltingpoint above 3600 F a low rate of solution in molten steel, and a verylow solubility in the continuous casting slag composition. Metals whichexhibit the foregoing properties and which can be used in the presentinvention include tungsten, molybdenum, niobium and tantalum.

It has been found that the best results are obtained when a relativelythin coating of the metal is applied having a thickness ranging betweenabout 0.002 and 0.020 inch. When the metal coating is substantiallybelow 0.002 inch thick significantly less protection is provided andwhen the meal has a thickness substantially greater than 0.020 inch,there is a tendency for the coating to spall from the surface of therefractory oxide.

It has also been found that when coating a fused refractory oxidenozzle, particularly a fused silica nozzle which has a vitreous internalstructure, the nozzle should not be allowed to heat to a temperaturewhich results in devitrifying the nozzle, since a devitrified nozzlewill shatter under normal operating conditions during the continuouscasting of molten steel.

When coating a fused silica continuous casting nozzle or the likevitreous structure with a surface coating of tungsten, molybdenum,niobium or tantalum where it is essential that the vitreous structurenot be heated to the devitrification temperature thereof, and where themelting points of each of the coating metals to be applied to thevitreous structure is substantially above the devitrificationtemperature of the structures, the coatings can not be applied by theusual process of dipping in a bath of the molten metal. It has beenfound, however, that a strong and tenacious coating of any of thecoating metals of the present invention can be applied to a fused silicanozzle or like vitreous structure without raising the temperaturethereof to its devitrification temperature by means of a plasmaspraying, vacuum vapor deposition, or sputtering process. The plasmaspraying process is particularly suited for coating the vitreousstructures of the present invention, because plasma spraying does notrequire vacuum operating conditions and provides very high temperaturesin the arc for melting the metal while permitting independent control ofthe surrounding atmosphere so that oxidation of the molten metalparticles formed can be minimized.

In the plasma spraying process which is the preferred method of applyingthe metal coatings in the present invention the metal, as wire orpowder, is melted in an inert atmosphere, such as argon, by anon-transferred plasma arc and propelled to the surface of a vitreousstructure, such as a fused silica continous casting nozzle, by the forceof the plasma jet. The droplets of molten metal upon striking thesurface of the fused silica having a multiplicity of microscopic surfacepores flatten out and anchor in the surface irregularities of the poresto form a continuous firmly adherent surface coating and provides a basefor building up additional layers of the droplets during the plasmaspraying process.

When a coating of molybdenum, for example, having a thickness of 0.010inch is applied to the outer surface of a fused silica continuouscasting nozzle in accordance with the above described preferred process,the coating adheres firmly to the fused silica surface when the nozzleis maintained in contact with a molten slag having a high solubility foroxides during the continuous casting of steel and while the coating isheated to an elevated temperature, despite the fact that the silicanozzle exhibits very little, if any, thermal expansion which could beexpected to cause the molybdenum coating to break away from the surfaceof the silica nozzle.

Similar deposits of the coating metals are formed by the vapordeposition and sputtering processes, because in each of the latterprocesses atoms or very small molten particles of the metal are formedand deposited on the surface of the unheated fused substrate in theabove described manner.

We claim:

1. In an apparatus for the continuous casting of a molten metalincluding a continuous casting mold, a tubular molten metal supplydevice for conducting molten metal from a supply source and dischargingthe molten metal below the surface of a pool of molten metal maintainedin the upper end of a said mold wherein in use said pool of molten metalhas on the surface thereof a protective layer of a molten slag comprisedof oxides and fluorides which has a high solubility for refractoryoxides, the improvement wherein said tubular molten metal supply deviceis formed of vitreous refractory oxides and has on the outer surfacethereof exposed to said layer of molten slag a protective surfacecoating of a refractory metal selected from the group consisting oftungsten, molybdenum, niobium and tantalum.

2. In the apparatus of claim 1, the further improvement wherein saidvitreous refractory oxide material is fused silica.

3. In the apparatus of claim 1, the further improvement wherein saidsurface coating metal is tungsten.

4. In the apparatus of claim 1, the further improvement wherein saidsurface coating metal is molybdenum.

5. In the apparatus of claim 1, the further improvement wherein saidsurface coating metal is niobium.

6. In the apparatus of claim 1, the further improvement wherein saidsurface coating metal is tantalum.

7. In the apparatus of claim 1, the further improvement wherein saidsurface coating of metal has a thickness between about 0.002 and 0.020inch.

8. A method of increasing the resistance of a Vitreous molten metalsupply device formed of a fused refractory oxide material against attackby a molten layer of slag having a high solubility for oxides when saidsupply device is maintained in contact with said slag within a castingmold which comprises; plasma spraying the surface of a fused vitreousrefractory oxide molten metal supply device at least in the area wheresaid supply device is adapted to contact said molten layer of slag, withan adherent coating of a metal selected from the group consisting oftungsten, molybdenum, niobium and tantalum.

9. A method as in claim 8, wherein said coating as applied has a coatingthickness between about 0.002 and 0.020 inch.

10. A method as in claim 8, wherein said fused vitreous refractory oxideis fused silica.

11. A method as in claim 8, wherein said surface coating metal istungsten.

12. A method as in claim 8, wherein said surface coating metal ismolybdenum.

13. A method as in claim 8, wherein said surface coating metal isniobium.

14. A method as in claim 8, wherein said surface coating metal istantalum.

References Cited UNITED STATES PATENTS 2,839,292 6/1958 Bellamy 26634 PT3,243,313 3/1966 Aves 117105.2 2,588,421 3/1952 Shepard 1l7105.2

OTHER REFERENCES Hall: Product Engineering, p. 59, 117-1052, vol. 36,No. 25, December 1965.

WILLIAM D. MARTIN, Primary Examiner J. H. NEWSOME, Assistant ExaminerU.S. Cl. X.R.

11793.l PF, 107, 123 B; 266-34 PT

